Method of and means for detecting impulse corona and preventing flashover due to impulse corona in lightning arresters



Aug. 6, 1957 ELDRIDG T. l. METHOD OF' AND MEANS FOR DETECTING IMFULSECORONA AND PREVENTING FLASHOVER DUE TO IMPULSFJ CORONA IN LIGHTNINGARRESTERS Filed Oct. 29, 1953 I 2 Sheets-Sheet 1 j3d" I \J6J IA'VENTO'R;EusnAs LELDRuocE H/s @from/ey Aug- 6, 1957 T. l. ELDRIDGE METHOD OF' ANDMEANS FOR DETECTING IMPULSE CORONA AND PREVENTING FLASHOVER DUE TOIMPULSE CORONA 1N LxGHTNING ARRESTERS 2 Sheets-Sheet 2 Filed OCT.. 29,1953 TAUslAs I. ELDRnocE HIS 0 TTO/l/EV 2,802,175 METHOD OF AND MEANSFOR DE-rnorlNG 1M- PULsE CORONA AND rnavnNrrNc FLASH- OVER DUE T0IMPULSE CRGNA IN LIGHT- NING-ARRESTERS Tausias I. Eldridge, Havertown,Pa., assignor to H. K. Porter Company, a corporation of DelawareApplication October 29, 1953, Serial No. 389,687

14 Claims. (Cl. 324-54) My invention is a method ofand means fordetecting and overcoming a tendency of lightning arresters to flashoverand fail as a result of the formation therein of an impulse corona, asdistinguished from ordinary corona resulting from system voltage stress.

My invention is applicable to lightning arreste'rs having electrodesforming arc gaps in series and including a charaoteristic element whichpermits flow of discharge current to earth, limits the voltage at theline terminal, and suppresses follow current.

Such an arrester is ordinarily so designed that the voltage appearingacross its terminals is normally balanced and distributed in directproportion to the :several irnpedances comprised in the whole, andconducting members of one polarity are separated radially orrectilineally from conducting members of opposite polarity by aninsulating tube which forms a part of the characteristic element.

In the use ofv such an arrester, a series gap electrode is connectedwith'a line terminal and a characteristic element electrode, of oppositepolarity from the line terminal electrode, is connected with ground. Thedischarge path through thecharacteristic element interposed between theline electrode and the ground electrode has a much lower impedance thanthe flashover path around the characteristic element from any conductingmember of one polarity to a conducting member of opposite polarity.

Preferably the characteristic element consists of a hard fibre protectortube containing narrow, elongated arc slots, yand encircled by agrounded grading ring. The arc slots may be formed by a series ofconcentric, cylindrical walls of gas forming material with -spacesbetween them forming multiple, parallel cylindrical arc slots eachhaving the shape of a narrow rectangular slot bent concentrically-around and spaced from Ian axis. Electrodes are' positioned at the endsof the slots and connected respectively with a series gap electrode andwith ground.

Such lightning arresters are commonly referred to as protector tubes orexpulsion arresters and examples thereof are shown in the Eldridge etal. U. S. Patent No. 2,619,610, and in the Eldridge Vet al. applicationSerial No. 223,089, on which Letters Patent issued December 29, 1953,No. 2,664,518.

Such lightning arresters, when well designed and made, are highlyeicient and dependable and the `occasional flashover before sparkoverresulting in failure of such well designed and well made arresters havebeen generally regarded as inexplicable.

I have discovered that such failures result from the spread of impulsecorona from one conducting member to another conducting member ofopposite polarity within the arrester just before sparkover. Suchimpulse corona emanates primarily from the region adjacent to the lineend of the arc slots-and particularly from the adjacent end ofthegrading ring when such ring is used. A corona of su'liicient intensityto reach a series gap electrode and cause ilashover through airimpinging on a smooth surfaced States Patent O 2,802,175 Patented Aug.6, 1957 ICC fibre protector tube results from the application across thearrester terminals of voltage of practically square wave front, positivepolarity and crest voltage just below (as well as at or above) thatrequired for sparkover in the absence 0f such corona. Only under specialconditions will other .types of impulse surge cause ashover before:sparkover and l have found no instance where a ilashover beforesparkover could be caused by other types of surge in an arrester immuneto ilashover before sparkover on the application of an equal voltage ofsquare wave front and positive polarity.

I have further discovered that the formation and extent of such impulsecorona is detectable by positioning sheet material having a surfacecoated with a photographically sensitive emulsion in juxtaposition tothe exterior of the characteristic element and between conductingmembers of Opposite polarities, e. g. a characteristic element electrodeand a series gap electrode, and discharging across the arresterterminals a voltage of very steep wave front, positive polarity and acrest just below normal sparkover voltage. The photographic developmentof the emulsion reveals a feathery design generally resembling frondshaving their bases adjacent to the top of the grading ring, or at thelevel of the upper electrode of the characteristic element.

It would appear that positive and negative charges are produced in thehard fibre tube, between conducting members of opposite polarities, andthat the electrostatic field due -to the voltage stress causes thesecharges to move more or less rapidly in the hard fibre, lthereby causingchanges in current-s in the dielectric and a tendency to upset thevoltage distribution or balance and the' eman'ation of corona with itsbase in the vicini-ty of the electrode on the line side of thecharacteristic element.

The grading ring, when used, is of opposite polarity from the electrodeon the' ground side o'f the series gap and facilitates the establishmentof an electrostatic field and the frond images produced when a gradingring is used are generally larger and denser than the images producedwhen no grading ring is employed.

I have further discovered that the formation of impulse corona betweenthe grading ring and the electrode of opposite polarity may be minimizedand its deleterious effects eliminated by preventing free passage of theimpulse corona along the surface of the 'hard fibre tube. This may beeffected by knurling the surface of the hard libre tube between the ringand electrode members or by encircling the libre tube and the upperportion of the grading ring with a strong, elastic insulator immune tothe effects of ozone, such as a polyvinyl chloride or soft rubber tubeor a tightly Wound polyvinyl chloride or soft rubber tape, whichconstantly exerts inward radial pressure.

The principles of my invention and the bes-t methods contemplated by meof applying such principles will furither appear in the followingdescription and the accompanying drawings in illustration thereof.

In the drawings, Fig. l is a lightning arrester equipped for thedetection of impulse corona in accordance with my invention; Figs. 2 and3 show enlarged views of the unrolled and developed photographic filmsencircling the upper portion of the characteristic element in Fig. 1after such films have been subjected to impulse corona generated byapplying across the arrester terminals a surge voltage just lbelownormal sparkover voltage of the arrester and of steep wave front andpositive polarity across the arrester terminals; Fig. 4 shows thearrester of Fig. l

equipped with photographic films for detecting impulse corona and with asleeve preventing the generation of impulse corona of suflicientintensity to reach from the grading ring to a series gap electrode;Figs. 5 and 6 show the unrolled and developed photographic films of Fig.4 after applying across the arrester terminals of Fig. 4 the samevoltages that produced the imagery of Figs. 2 and 3 when applied acrossthe terminals of Fig. 1; Fig. 7 1s afragmentary View of a portion of thearrester shown 1n Fig. l (with films omitted) after wrapping the exposedprotector tube and part of the grading ring with tightly drawn layers ofpolyvinyl chloride or soft rubber tape; and Fig. 8 is a fragmentary Viewof a portion of the arrester shown in Fig. l (with films omitted) afterknurling the exposed external periphery of the potector tube.

The arrester shown in the drawings comprises a wet process porcelain,cylindrical, hollow ,body 1 having its top closed by a porcelain plug 2which supports a line terminal 3 from which depends a series gapelectrode 4.

A flanged, threaded conducting cup 6 is cemented in the countersunkbottom of the body v1 and terminates in a ground conductor '7 containinga discharge vent S for arc slots hereinafter described. The flanges ofthe cup 6 and the cement securing the same contain vent passages 9 forVentilating the interior of the body 1.

A hard fibre tube 10 is threaded into the cup 6 and tightly encircled`by a grounded grading ring 11 consisting of a steel reinforcing sleeveconductively seated in a `groove in the top of the cup 6.

A circular, -grounded arc electrode 12 is conductively seated in the cup6 and supports a hard fibre tube 13 in close, radially spacedrelationship to `the tube 10. The electrode 12 also supports a hardfibre rod 14 in close, radially spaced relationship to the tube 13. Theupper ends of the tube 13 and rod 14 support, and are radiallypositioned by, an intermediate electrode 1S whose periphery makes aclose fit with the interior of the libre tube 10 at approximately thelevel of the top of the grading ring 11. The space between the tubes 1t)and 13 and the space between the tube 13` and rod 14 form elongated arcslots 16 and 16 of narrow cross section and of unequal lengths betweenthe electrodes 15 and 12. The electrode 12 contains apertures 17 throughwhich such slots 16, 16 discharge to the vent 8.

A conducting metal tube 18 is sleeved on a boss on 'the top of theelectrode 15 and on a boss on the bottom of a conducting metallic plug19 which is threaded into the top of the fibre Itube 10. An electrode 19is adjustably seated in a socket in the top of the plug 19 and formswith the electrode 4 a series gap 2t) for normally isolating theprotector tube from line voltage.

The application of sparkover voltage of usual shape and polarity acrossthe arrester terminals sets up an electrostatic field which causessufficient ionization of air in the discharge path to cause sparkoveracross the isolating gap 20 and through the arc slots 16 and 16 andpermit discharge of the surge to ground. The ionization of the air inthe discharge path also permits follow current to flow to ground fromthe system and maintain arcs across the gaps. The action of the arcs onthe hard fibre walls of the arc slots 16, 16' generates deionizing gasand builds up high pressures in these slots. The deionizing particlesand pressure effect suppression of the arcs in the slots 16, 16 andconsequent quenching of the arc across the isolating gap 20.

By my invention the liability to failure of a particular design ofarrester under extreme conditions may be detected by encircling theupper portion of the grading ring 11 and of the protector tube il@ withfilms 21 and 2,2 coated with a sensitive photographic emulsion. Suchemulsion may be of the type ordinarily used for photographing in blackand white contrast or of the type used to obtain color contrast. Thereis then impressed across the arrester lterminals an impulse or surgevoltage which is preferably just `below normal sparkover voltage and ofsteep, e. g. substantially square, wave front and positive polarity. TheVoltage stresses involving a voltage wave of positive polarity generatemore intense impulse `corona than those of a voltage wave of negativepolarity, and the steeper the wave front the shorter is the timeavailable for the actions which must take place in initiating sparkoverof the arrester. An arrester, which will withstand ashover under theforegoing conditions, will not ashover upon application of surges ofnegative polarity and of more gradual wave front even though of highervoltages, and will sparkover and perform its intended functions uponapplication of sparkover voltage of any shape or polarity.

The greater the steepness of the voltage wave, the shorter is the timeto sparkover and the higher is the voltage needed to initiate sparkover.For example, a 500 kv. voltage wave having a rising front averaging 100kv. per microsecond ywhen reaching an arrester that normallysparkover'at 75 kv. will reach 75 kv. in about threefourths of amicrosecond and if for any reason sparkover does not occur immediately,the continued rise creates a stress resulting in an impulse corona whichdevelops in a fractional part of a millionth of a second and isditiicult to identify. As the ltime period approaches zero, the moreimportant become the coincident effects and/ or secondary actions causedby the stress, and particularly the generation of impulse corona in theregion of the protector tube encircling the electrode at the upper endof the arc slots. Such impulse corona is not to 1be confused with coronaof the steady state type such as might be produced by system voltageadjacent to the isolating gap.

Since the crest Value of voltage required to initiate sparkover of thearrester gaps increases with the steepness of front of the Voltage wave,viz. the time required yfor the wave to rise from zero to crest, a veryintense corona normally results from a voltage of square wave front andpositive polarity.

The space relationship of the intermediate electrode 15 with thegrounded grading ring 11 and the separation of these parts by the outerfibre tube 10 helps to develop an intense electric field and facilitatesfast sparkover from the outer intermediate electrode 15 to the terminalelectrode 12 through the are slots 16, 16', since the dielectricstrength of the tube 19 prevents a discharge from passing therethrough.

Since the intermediate electrode 15, plug 19 and series gap electrode 19are conductively connected together, the same difference of potentialwhich establishes stress between the intermediate electrode 15 and thegrading ring 11 will also exist between the plug 19 and grading ring 11along the smooth, outer surface of the tube 10 of Fig. 1, hence thedistance between these parts is always made sufficiently long to providea factor of safety sufciently high to prevent discharge, under normalsparkover conditions, from the plug 19 to the grading ring 11 instead offrom the intermediate electrode 15 through the slots 16, 16 to theterminal electrode 12'.

When, however, a surge voltage of positive polarity and practicallysquare wave front is applied across the line and ground terminals of anarrester containing photographically sensitive lms 21 and 22 (with dueprecautions to prevent any elect by such media on the electric field andcurrent development) and the voltage wave reaches crest value inpractically zero time, an impulse corona is generated which extendsupward from the tube zone surrounding the electrode 1S and activates thephotographic emulsion to form thereon images which, whenphotographically developed in the usual manner, generally have the formof branches or fronds, e. g. as shown in Figs. 2 and 3. Fig. 2 shows atypical design produced on the upper film of Fig. 1 and Fig. 3 shows atypical design produced on the lower film of Fig. 1 when an arrester issubject to voltage slightly below normal sparkover voltage and ofpositive polarity and substantially square wave front, with the gradingring 11 in place. If the grading ring is omitted, the images produced onthe upper and lower films more nearly resemble scattered clumps of grassor ferns. Where colored film is used, the shorter fronds usually appearin blue color whereas the longer fronds usually appear in yellow color.If the volt- 'age sfraised lto-or about normal sparkover voltage,hashover occurs before sparkover and the film isseared. If the steepnessof the wave front is dimlnished the imagesdecrease in density andamplitude and generally disappear from the upper 'film even though thevoltage be increased. j

I believe ,that the `generation of impulse corona produces positiveandnegative charges in the hardibre tube and that such charges tend to movemore or less rapidly and 'cause changes in currents in the tube 10,resulting in a tendency to upset the voltage distribution or balancebetween the parts of the arresterV and particularly between theplug 19and the grading ring 11`.

vThe generation of impulse corona of limited intensity might not be tooserious, but in conjunction therewith the grading ring 11 tends tofacilitate the establishment of a flashover dueto the mobility of ionsdischarged from its sharp edge as a secondary transmitter due to rapidmovement of charges within the adjacent tube 10 under the inuence of thealmost instantaneous establishment of the electric eld.

Circuit constants of distributionlines tend to determine the steepnessof a wave front that travels along the circuit and square fronts* areless likely to be present than fronts y controlof impulse corona and itseffects under such con-` ditions will eliminate ashover underconditionsinvolving the discharge of the more prevalent surges ofnegative polarity and gradual slope of the wave front.

I have found that the emanation of impulse corona from the grading ring11 and from the tube periphery adjacent thereto may be minimized, andits ilashover effects eliminated, by covering the upper portion of thegrading ring 11 and the smooth surface of the tube 10 between thegrading ring 11 and plug 19 with a medium of high dielectric strengthwhich constantly exerts inward radial pressure on the outer periphery ofthe tube 10 and the upper portion of the grading ring 11, as shown inFigs. 4 and 7.

As shown in Fig. 4, the upper portion of the grading ring 11 and theperipheral surface of the tube 10 are encased in constricting tubing 25of polyvinyl chloride, rubber, or the like. The tubing is normally ofslightly smaller diameter than the outside diameter of the tube 10, andis expanded and slipped over the tube 10 and ring 11 and then allowed toshrink. Such tubing 25 should be of suicient thickness and length toprevent the formation of any images extending all the way from thegrading ring 11 to the plug 19 on the photographically sensitive films21a, 22a when a surge voltage of positive polarity and substantiallysquare wave front is impressed across the terminals 3 and 7 of thearrester. The absence of images across the height of the upper film 21a,when the arrester has voltage of square wave front and positive polarityimpressed across its terminals as heretofore described, is a determinantthat the thickness, length and quality of the tubing is suicient toprevent ashover before sparkover under any conditions.

In lieu of the tubing 25, the upper portion of the grading ring 11 andthe peripheral surface of the tube 10 may be tightly wrapped in layersof polyvinyl chloride or soft rubber tape 26 so as to form a tightsleeve of sufiicient thickness, length and constrictiveness to preventthe production of images on a photographically sensitive film positionedas shown at 21 and 21a in Figs. l and 4, but omitted from Fig. 7.

I have further found that knurling or forming closely spacedtransversely extending narrow grooves 27 in the peripheral surface ofthe upper portion of the tube 10 will give substantially the sameprotective effect against the upward travel of impulse corona as theapplication lof the tight sleeves shown in Figs. 4 and 7.

In all cases it is desirable tocontrol the circulation of air and ionsbetween the tube 10 and the inner wall of the body 1 by inserting layersof glass libre batting 28, but this in itself is insucient 'to preventthe formation and upward' travel of impulse corona and ashover underconditions of surge voltage of positive polarity and very steep wavefront.

Having described my invention, I claim:

I. A method of detecting impulsecorona in a lightning arrester having aprotective tube including-series gap electrodes and arc slot electrodes,said series gap electrodes being spaced by a hard libre tube from one ofsaid arc slot electrodes, which comprises encircling said tube withmaterial having a surface coated with a photographically sensitiveemulsion, and impressing across said arrester an impulse voltage ofpositive polarity and substantially square wave front.

2. A methodl of detecting impulse corona in a lightning arrestercomprising a protective tube having series gap electrodes and arc slotelectrodes which comprises positioning material having a surface coatedwith a photographically sensitive emulsion between one of said seriesgap electrodes and one of said arc slot electrodes, and impressingacross the arrester a surge voltage of substantially square wave front.

3. A method of detecting impulse corona in a lightning arrester havingseries gap electrodes and a characteristic element containing spacedelectrodes which comprises positioning-material having a surface coatedwith a photographically sensitive emulsion adjacent to saidcharacteristic element between one of the electrodes thereof and one ofsaid series gap electrodes, and impressing across said arrester animpulse voltage of steep wave front.

4. A lightning arrester comprising a protector tube including series gapelectrodes and arc slot electrodes, and material having a surface coatedwith a photographically sensitive emulsion bent around said tube betweenone of said series gap electrodes and one of said arc slot electrodes.

5. A lightning arrester comprising a protector tube having series gapelectrodes and arc slot electrodes, and means for preventing theactivation of a photographically sensitive emulsion on sheet materialpositioned adjacent to said tube between one of said series gapelectrodes and one of said arc slot electrodes when an impulse voltageof positive polarity and substantially square wave front is impressedacross the arrester.

6. A lightning arrester comprising a protector tube having series gapelectrodes and arc slot electrodes and a constricting sleeve ofnon-conducting material encircling said tube between one of said seriesgap electrodes and one of said arc slot electrodes and of sufficientthickness to prevent activation of a photographically sensitive emulsionon sheet material adjacent thereto when a surge voltage of positivepolarity and substantially square wave front is impressed across saidarrester.

7. A lightning arrester comprising a protector tube including series gapelectrodes and arc slot electrodes, a grounded grading ring surroundingsaid protector tube and a constricting sleeve of non-conducting exibleelastic material immune to the effects of ozone encircling at least apart of said grading ring and encircling said tube from said gradingring to one of said series gap electrodes, said constricting sleeveexerting inward pressure throughout the length thereof on said tube andgrading ring.

8. A lightning arrester comprising a protector tube including series gapelectrodes and arc slot electrodes, the outer surface of said tube beingknurled between one of said series gap electrodes and one of said arcslot electrodes.

9. A method of testing a lightning arrester having electrodes formingarc gaps and having a characteristic element which comprises applyingacross the arrester terminals a surge voltage slightly below normalarcover voltage' and having a substantially square wave front and iincluding a series gap electrode and an arc slot electrode between whichimpulse corona tends to, form on the dischargeY between them of positivesurges having steep wave fronts, and an expanded tube of elasticpolyvinyl chloride tightly encircling said tube substantially the entiredistance between said electrodes and exerting inward pressure throughoutthe length of said expanded tube.

l1. A lightning arrester comprising a protective tube including a seriesgap electrode and an arc slot electrode between which impulse coronatends to form on the discharge between them of positive surges havingsteep wave fronts, and an expanded tube of rubber tightly encirclingsaid tube substantially the entire distance between said electrodes andexerting inward pressure throughout the length of said expanded tube.

12. A lightning arrester comprising a protective tube including a seriesgap electrode and an arc slot electrode between which impulse coronatends to form on the discharge between them of positive surges havingsteep wave fronts, and a band of polyvinyl chloride tightly wound instretched condition around said tube substantially the entire distancebetween said electrodes, said band exerting inward pressure throughoutthe winding thereof.

13. A lightning arrester comprising a protective tube including a seriesgap electrode and an arc slot electrode between which impulse coronatends to form on the '8 dischargejbetween'them of positive surgeshaving-steep wave fronts, and a band of soft rubber tape tightly woundin' stretched condition around said tube substantially the entiredistance between said electrodes, said 'tape exerting inward pressurethroughout the winding thereof.

14. A lightning arrester comprising a protective tube including seriesgap relectrodes and arc slot electrodes, and a Anon-conducting sleeve offlexible elastic material immune to the effects of ozone and selectedfrom the group consisting of polyvinyl chloride and soft rubberencircling said tube throughout substantially the entire distancebetween the gap forming portion of one of said series vgap electrodesand one of said arc slot'electrodes; said sleeve exerting inwardpressurethroughout the length thereof.

ReferencesCited in the le of this patent UNITED STATES PATENTS 1,957,306vogel May 1, 1934 2,170,813 Doble a- Aug. 29, 1939 2,338,479 AckermannJan. 4, 1944 2,372,035 Wade Mar. 20, 1945 2,603,685 Bychinsky July l5,1952 2,677,072 De Val Apr. 27, 1954 OTHER REFERENCES Enclosed SparkGaps, pages 429-432, American Institute of Electrical EngineersTransactions, volume 59, August 1940.

